Integrins are a superfamily of cell adhesion receptors, which exist as heterodimeric transmembrane glycoproteins. They are part of a large family of cell adhesion receptors which are involved in cell-extracellular matrix and cell-cell interactions. Integrins play critical roles in cell adhesion to the extracellular matrix (ECM) which, in turn, mediates cell survival, proliferation and migration through intracellular signaling. The receptors consist of two subunits that are non-covalently bound. Those subunits are called alpha (α) and beta (β). The alpha subunits all have some homology to each other, as do the beta subunits. The receptors always contain one alpha chain and one beta chain and are thus called heterodimeric. Both of the subunits contribute to the binding of ligand. Eighteen alpha subunits and eight beta subunits have been identified, which heterodimerize to form at least 24 distinct integrin receptors.
Among the variety of alpha chain subunits is a protein chain referred to as alpha V. The ITGAV gene encodes integrin alpha chain V (vitronectin receptor, alpha-v; αv, antigen CD51). The I-domain containing integrin alpha-v undergoes post-translational cleavage to yield disulfide-linked heavy and light chains, that combine with multiple integrin beta chains to form different integrins. Alternative splicing of the gene yields seven different transcripts; a, b, c, e, f, h, j altogether encoding six different protein isoforms of alphaV. Among the known associating beta chains (beta chains 1,3,5,6, and 8; ‘ITGB1’, ‘ITGB3’, ‘ITGB5’, ‘ITGB6’, and ‘ITGB8’), each can interact with extracellular matrix ligands. The alpha V beta 3 integrin, perhaps the most studied of these, is referred to as the vitronectin receptor (VNR). In addition to providing for cell attachment to other cells or to extracellular proteins such as vitronectin (αvβ3) and fibronectin (αvβ6), the integrins are capable of intracellular signaling which provides clues for cell migration and secretion of or elaboration of other proteins involved in cell motility and invasion and angiogenesis. The alpha-v integrin subfamily of integrins recognize the ligand motif arg-gly-asp (RGD) present in fibronection, vitronection, VonWillebrand factor, and fibrinogen. The alpha-V integrins are receptors for vitronectin, cytotactin, fibronectin, fibrinogen, laminin, matrix metalloproteinase-2, osteopontin, osteomodulin, prothrombin, thrombospondin and von Willebrand factor. In case of HIV-1 infection, the interaction with extracellular viral Tat protein seems to enhance angiogenesis in Kaposi's sarcoma lesions.
It has been established that integrins which are alpha-v containing heterodimers, particularly alpha-v/beta-6, the receptor for fibronectin, are involved in adhesion of carcinoma cells to fibronectin and vitronectin. This is especially true, for carcinoma cells arising from the malignant progression of colon cancer (Lehmann, M. et al., Cancer Res., 54(8):2102-7 (1994)). Furthermore, integrin expression in colon cancer cells is regulated by the cytoplasmic domain of the beta-6 integrin subunit which signals through the ERK2 pathway (Niu, J. et al., Int. J. Cancer, 99(4): 529-537 (2002)) and beta6 expression is associated with secretion of gelatinase B, an enzyme involved in tumor cell invasion and metastatic mechanisms (Agrez et al., Int. J. Cancer, 81(1):90-97 (1999)).
There is now considerable evidence that progressive tumor growth is dependent upon angiogenesis, the formation of new blood vessels, to provide tumors with nutrients and oxygen, to carry away waste products and to act as conduits for the metastasis of tumor cells to distant sites (Gastl, G. et al., Oncol., 54(3):177-184 (1997)). Recent studies have further defined the roles of integrins in the angiogenic process. During angiogenesis, a number of integrins that are expressed on the surface of activated endothelial cells regulate critical adhesive interactions with a variety of ECM proteins to regulate distinct biological events such as cell migration, proliferation and differentiation. Specifically, the closely related but distinct integrins αvβ3 and αvβ5 have been shown to mediate independent pathways in the angiogenic process. An antibody generated against αvβ3 blocked basic fibroblast growth factor (bFGF) induced angiogenesis, whereas an antibody specific to αvβ5 inhibited vascular endothelial growth factor (VEGF) induced angiogenesis (Eliceiri et al., J. Clin. Invest., 103:1227-1230 (1999); Friedlander et al., Science, 270:1500-1502 (1995)). Therefore, integrins, and especially the alpha V subunit containing integrins, are a therapeutic targets for diseases that involve angiogenesis, such as diseases of the eye and neoplastic diseases, tissue remodeling such as restenosis, and proliferation of certain cells types, particularly epithelial and squamous cell carcinomas.
Liposomes are spherical vesicles comprised of concentrically ordered lipid bilayers that encapsulate an aqueous phase. Liposomes serve as a delivery vehicle for therapeutic agents contained in the aqueous phase or in the lipid bilayers. Delivery of drugs in liposome-entrapped form can provide a variety of advantages, depending on the drug, including, for example, a decreased drug toxicity, altered pharmacokinetics, or improved drug solubility. Liposomes when formulated to include a surface coating of hydrophilic polymer chains, so-called Stealth® or long-circulating liposomes, offer the further advantage of a long blood circulation lifetime, due in part to reduced removal of the liposomes by the mononuclear phagocyte system. Often an extended lifetime is necessary in order for the liposomes to reach their desired target region or cell from the site of injection.
Targeted liposomes have targeting ligands or affinity moieties attached to the surface of the liposomes. The targeting ligands may be antibodies or fragments thereof, in which case the liposomes are referred to as immunoliposomes. When administered systemically targeted liposomes deliver the entrapped therapeutic agent to a target tissue, region or, cell. Because targeted liposomes are directed to a specific region or cell, healthy tissue is not exposed to the therapeutic agent. Such targeting ligands can be attached directly to the liposomes' surfaces by covalent coupling of the targeting ligand to the polar head group residues of liposomal lipid components (see, for example, U.S. Pat. No. 5,013,556). This approach, however, is suitable primarily for liposomes that lack surface-bound polymer chains, as the polymer chains interfere with interaction between the targeting ligand and its intended target (Klibanov, A. L., et al., Biochim. Biophys. Acta., 1062:142-148 (1991); Hansen, C. B., et al., Biochim. Biophys. Acta, 1239:133-144 (1995)).
Alternatively, the targeting ligands can be attached to the free ends of the polymer chains forming the surface coat on the liposomes (Allen. T. M., et al., Biochim. Biophys. Acta, 1237:99-108 (1995); Blume, G. et al., Biochim. Biophys. Acta, 1149:180-184 (1993)). In this approach, the targeting ligand is exposed and readily available for interaction with the intended target.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.